Carrier Aggregation will be used in future LTE networks to provide improved data rates to users. Carrier aggregation consists of transmitting data to or receiving data from the UE on multiple carrier frequencies (“component carriers”). The wider bandwidth enables higher data rates.
A UE can be configured with a set of component carrier (CCs). Specifically, the UE is configured with a cell on each component carrier. Some of these cells may be activated. The activated cells can be used to send and receive data (i.e., the activated cells can be used for scheduling). The UE has up to date system information for all configured cells. Therefore, after a cell has been configured, it can be quickly activated. Thus, when there is a need for aggregating multiple CCs (e.g., a large burst of data), the network can activate configured cells on one or more of the CCs. There is a designated primary serving cell (Pcell) on a CC that is referred to as the primary CC, which is always activated. The other configured cells are referred to as secondary serving cells (Scells), and the corresponding CCs are referred to as secondary CCs.
Remote Radio Heads (RRHs) are used to extend coverage of a base station. As part of the work on carrier aggregation, next-generation cellular communication systems will support carrier aggregation of frequencies on which RRHs are deployed. Carrier Aggregation will be used to provide improved data rates to users. Carrier aggregation consists of transmitting data to or receiving data from the user equipment (UE) on multiple carrier frequencies (“component carriers”). The wider bandwidth enables higher data rates.
RRHs are deployed on a different frequency than the frequency used by the base station site and provide hot-spot like coverage on that frequency. User equipment (UE) that is in such a hot-spot can perform carrier aggregation of the frequency used by the base station and the frequency used by the RRH and obtain corresponding throughput benefits. RRHs do not embody typical base station functionalities such as higher layer processing, scheduling etc. The baseband signal transmitted by an RRH is generated by the base station and is carried to the RRH by a high speed wired (e.g., optical) link. Thus RRHs function as remote antenna units of a base station, with a high speed link to the base station.
A base station 101, RRH 102, and UE 103 are shown in FIG. 1. As is evident, a non-wireless link 104 exists between base station 101 and RRH 102. The transmissions to UE 102 occur both from base station 101 and from RRH 102, except that the transmissions from base station 101 exist on a different frequency than the transmissions from RRH 102.
The presence of RRHs introduces additional physical locations from which the UE can receive the base station signal (i.e., in addition to receiving the base station signal directly from the base station). In addition, there is a delay introduced by the communication between the base station and the RRH. This delay results in the UE perceiving very different propagation delays on the frequency used by the base station and the frequency used by the RRH. As a consequence, the timing advance applied to the two frequencies need to be different.
FIG. 2 shows the timing relationships between downlink and uplink transmissions of the two frequencies. In particular, downlink (DL) transmission (Tx) is shown on frequency 1 (F1) as subframe 201, DL reception (Rx) is shown on F1 as subframe 202, UL Tx is shown on F1 as subframe 203, UL Rx is shown on F1 as subframe 204. In a similar manner DL Tx is shown on F2 as subframe 205, DL Rx is shown on F2 as subframe 206, UL Tx is shown on F2 as subframe 207, and UL Rx is shown on F2 as subframe 208.
It is assumed that base station 101 tries to ensure that uplink transmissions on F1 and F2 are received at the same time. Transmissions on F2 through RRH 102 (both uplink and downlink) have an additional delay due to transmission through fiber link 104 and the associated RRH processing. This additional delay can be as large as 30 microseconds. As shown in FIG. 2, in order for the F2 uplink to arrive at the base station at the same time as the F1 uplink, the timing advance applied by the UE for transmissions on F2 has to compensate for the fiber and RRH processing delay.
As a result, the uplink subframes 203, 204, 207, and 208 on F1 and F2 are not time aligned. In FIG. 2, F2 uplink subframe 207 starts before F1 uplink subframe 203.
There may also be a need for a different frame timing when the two carriers are on different bands with a large frequency separation, even without deployment of RRHs. In such cases the UE has to maintain a separate timing advance for the second carrier.
In order to obtain a timing advance for the primary carrier, the UE has to perform a random access procedure on the primary carrier. The UE transmits a random access preamble to the eNB on the primary uplink carrier frequency. The eNB calculates the appropriate timing advance the UE should apply based on the timing of the received random access preamble. The eNB transmits a Random access response message (RAR) in response to a RACH preamble transmission by the UE, on the primary downlink carrier frequency. The RAR includes the timing advance calculated by the eNB from the RACH transmission.
In order to maintain uplink timing on a secondary carrier, the UE has to perform a random access preamble transmission on the secondary carrier. Transmitting the RAR message on the downlink of the secondary carrier is a possibility. However, such a mechanism is inadequate, as discussed below.
Support of heterogeneous network scenarios: If an LTE UE is experiencing control channel interference on the SCell (for example, due to presence of interfering pico cells on the secondary CC), then the UE is unable to receive PDCCH transmission from the eNB on the SCell. Receiving the RAR message requires the UE to be able to receive the PDCCH for the RAR message, which the UE is unable to do in this case. In such a case, the eNB configures “cross carrier scheduling” for the SCell: the PDCCH for the SCell is transmitted on a different cell (e.g., PCell). This enables the eNB to use the SCell for PDSCH transmissions. However, since there isn't a mechanism to resynchronize uplink timing, the UE will be unable to use the SCell uplink.
Additional PDCCH blind decodes: In LTE, the downlink control information is transmitted via the Physical Downlink Control channels (PDCCH). The PDCCH typically contains control information about the downlink control information (DCI) formats or scheduling messages, which inform the UE of the modulation and coding scheme, transport block size and location, precoding information, hybrid-ARQ information, UE Identifier, Carrier Indicator Function, CSI request fields, SRS request field, etc. that is required to decode the downlink data transmissions or for transmitting on the uplink. This control information is protected by channel coding (typically, a cyclic-redundancy check (CRC) code for error detection and convolutional encoding for error correction) and the resulting encoded bits are mapped on the time-frequency resources. For example, in LTE Rel-8, these time-frequency resources occupy the first several OFDM symbols in a sub-frame. A group of four Resource Elements is termed as a Resource Element Group (REG). Nine REGs comprise a Control Channel Element (CCE). The encoded bits are typically mapped onto either 1 CCE, 2 CCEs, 4 CCEs or 8 CCEs. These four are typically referred to as aggregation levels 1, 2, 4 and 8. The UE searches the different hypotheses (i.e., hypotheses on the aggregation level, DCI Format size, etc) by attempting to decode the transmission based on allowable configurations. This processing is referred to as blind decoding.
To limit the number of configurations required for blind decoding, the number of hypotheses is limited. For example, the UE does blind decoding using the starting CCE locations as those allowed for the particular UE. This is done by the so-called UE-specific search space (UESS), which is a search space defined for the particular UE (typically configured during initial setup of a radio link and also modified using RRC message). Similarly a common search space (CSS) is also defined that is valid for all UEs and might be used to schedule broadcast downlink information like Paging, or Random access response, or other purposes. The number of blind decoding attempts that a UE performs is limited (e.g. 44 in Rel-8 LTE (12 in CSS and 32 in UESS), and up to 60 for Pcell (12 in CSS, and 48 in UESS) and up to 48 for Scell (48 in UESS) in Rel-10) for several reasons.
Other than reducing the computational load (i.e. convolutional decoding attempts), limiting the number of blind decodes also help in reducing CRC falsing rate. A CRC falsing occurs when the UE decodes an incorrect transmission and treats it as a valid PDCCH because the CRC passes and this can lead to protocol errors or other errors resulting in system performance loss. Therefore, it is desirable to keep the falsing rates very low. Typically, for a k-bit CRC attached, if n is the number of decoding attempts, the probability of CRC falsing (i.e. false positive) is approximately n×2−k.
For carrier aggregation operation in LTE, the system information is typically transmitted via the Pcell and hence the UE monitors the CSS and UESS for the Pcell. The Scell system information is transmitted via RRC signaling (on a UE specific basis) and this can be transmitted via the Pcell and there is no need for the UE to monitor the CSS on the Scells. Thus, the CSS corresponding to the SCell are not monitored and the UE benefits from having to perform smaller number of blind decodes for the Scells. The RAR is transmitted using an RA-RNTI (i.e., the PDCCH for the RAR is scrambled using an RA-RNTI). RA-RNTI is a broadcast identifier and reception of the PDCCH for RAR requires monitoring of the common search space on the SCell DL. As described above, monitoring the common search space on the SCell DL requires additional blind decodes, which increases the complexity of the UE. Therefore a procedure to acquire SCell UL timing without requiring monitoring the PDCCH common search space on the SCell is needed.
There are two types of random access procedures—contention based random access (CBRA) and contention free random access (CFRA). In the contention based random access procedure, the UE selects a random access preamble and contention resolution is performed. The CBRA procedure is illustrated in FIG. 4. UE (401) transmits a random access preamble (411) to the eNB (402). The eNB computes the timing advance of the UE and transmits a Random access response (RAR) message (412). The RAR message includes the timing advance computed by the eNB and an uplink resource allocation (uplink grant). The UE transmits a contention resolution message (413) using the uplink resources granted in the RAR message. The eNB responds with a contention resolution message (414) which indicates whether contention resolution is successful. It is possible that two UEs select the same random access preamble and transmit the random access preamble at the same time. In this case, both UEs attempt to transmit their respective contention resolution messages (413) using the uplink grant. The contention resolution message (413) includes a unique identifier of the UE. If the eNB is able to decode both the contention resolution messages, it can select one of the two UEs as the winner of the contention resolution and transmit the contention resolution message (414) indicating the winner of the contention resolution. The other UE can reattempt the random access procedure.
The CFRA procedure is illustrated in FIG. 5. The eNB (502) transmits an RA preamble assignment message (510) to the UE (501). The RA preamble assignment message assigns a random access preamble to the UE. The UE transmits the assigned random access preamble (511) to the eNB. The eNB computes the timing advance of the UE and transmits a Random access response (RAR) message (512). The RAR message includes the timing advance computed by the eNB. Given that the eNB assigns the preamble to the UE, the eNB can ensure that there is a single UE using the preamble at a given time. Consequently, there is no need for contention resolution in the CFRA procedure.
Contention free random access (CFRA) requires the eNB to reserve more preambles to support the additional RACH transmissions for Scell RACH when UEs capable of uplink aggregation are present (in addition to RACH transmissions for handover, DL data arrival). In general, CFRA is helpful when it is important for the RACH procedure to complete quickly. For SCell RACH, quick completion is not as critical. Not having to reserve RACH preambles is a more critical requirement.
Thus there is a need to support contention based random access to obtain uplink timing on SCells, while also overcoming the deficiencies related to receiving the RAR message on the same cell on which the RACH preamble is transmitted.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP) executing software instructions stored in non-transitory computer-readable memory. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.